Soil methane emissions from plain poplar (Populus spp.) plantations with contrasting soil textures

The forest soil methane (CH4) flux exhibits high spatiotemporal variability. Understanding these variations and their driving factors is crucial for accurately assessing the forest CH4 budget. In this study, we monitored the diurnal and seasonal variations in soil CH4 fluxes in two poplar (Populus spp.) plantations (Sihong and Dongtai) with different soil textures using the static chamber-based method. The results showed that the annual average soil CH4 flux in the Sihong and Dongtai poplar plantations was 4.27 ± 1.37 kg CH4-C ha–1 yr–1 and 1.92 ± 1.07 kg CH4-C ha–1 yr–1, respectively. Both plantations exhibited net CH4 emissions during the growing season, with only weak CH4 absorption (–0.01 to –0.007 mg m–2 h–1) during the non-growing season. Notably, there was a significant difference in soil CH4 flux between the clay loam of the Sihong poplar plantation and the sandy loam of the Dongtai poplar plantation. From August to December 2019 and from July to August and November 2020, the soil CH4 flux in the Sihong poplar plantation was significantly higher than in the Dongtai poplar plantation. Moreover, the soil CH4 flux significantly increased with rising soil temperature and soil water content. Diurnally, the soil CH4 flux followed a unimodal variation pattern at different growing stages of poplars, with peaks occurring at noon and in the afternoon. However, the soil CH4 flux did not exhibit a consistent seasonal pattern across different years, likely due to substantial variations in precipitation and soil water content. Overall, our study emphasizes the need for a comprehensive understanding of the spatiotemporal variations in forest soil CH4 flux with different soil textures. This understanding is vital for developing reasonable forest management strategies and reducing uncertainties in the global CH4 budget.


Patterns of temperature, precipitation, and soil water content in two poplar forests
Throughout 2019-2020, trends in Ta, precipitation, Ts, and SWC exhibited consistent patterns in both the Sihong and Dongtai poplar plantations (Fig. 1a,b).In 2019, the highest temperatures in the Sihong and Dongtai poplar plantations were recorded in July (28.0 ℃ for Ta and 29.4 ℃ for Ts) and August (27.0℃ for Ta and 27.5 ℃ for Ts), respectively.In 2020, the peak temperatures in both plantations occurred in August, with Sihong reaching 29.1 ℃ for Ta and 28.6 ℃ for Ts, and Dongtai reaching 28.9 ℃ for Ta and 28.2 ℃ for Ts.Precipitation in both plantations exhibited a unimodal pattern.In 2019, the peak monthly cumulative precipitation occurred in August, with 145 mm in Sihong and 166 mm in Dongtai.In 2020, the peak monthly cumulative precipitation in both plantations occurred in July for Sihong (357 mm) and June for Dongtai (319 mm).
Throughout 2019-2020, the SWC in the Dongtai poplar plantation consistently exceeded that in the Sihong poplar plantation, likely due to slightly higher precipitation levels in Dongtai compared to Sihong.Generally, the SWC in both poplar plantations exhibited a seasonal trend, with lower levels during summer and higher levels during winter.Notably, during the summer of 2020, increased rainfall resulted in the SWC reaching its peak in July for both the Sihong and Dongtai poplar plantations, recording values of 0.29 and 0.40 cm 3 cm -3 , respectively.

Diurnal variations of soil CH 4 flux in two poplar forests
In the Sihong poplar plantation, during the early growing season (March 31 to April 1) and the rapid growing season (May 29 to 30), the soil exhibited CH 4 emissions throughout the day (ranging from 0.001 to 0.11 mg m -2 h -1 ), with peak emission rates observed at around 12:30 (0.06 and 0.11 mg m -2 h -1 ), followed by a gradual decline.From 18:00 until the next day at 08:00, the emission rate remained stable (Fig. 2a,b).During the late peak growing season (August 29 to 30), the soil emitted CH 4 throughout the day (0.001 to 0.05 mg m -2 h -1 ), peaking at around 12:00, and then exhibited a fluctuating and decreasing trend (Fig. 2c).During the non-growing season (December 15 to 16), the soil did not emit CH 4 but instead showed CH 4 absorption throughout the day (-0.05 to -0.0001 mg m -2 h -1 ), with fluctuations and a peak absorption rate at 12:30 (Fig. 2d).
Due to logistical challenges related to transportation and weather conditions, we were only able to obtain effective diurnal variation data for the Dongtai poplar plantation during the rapid growing season (May 13 to 14) and the non-growing season (December 10 to 11) (Fig. 2e,f).During the rapid growing season, the soil CH 4 emission rate increased from the early morning and reached a peak of 0.07 mg m -2 h -1 at 14:30.It then gradually decreased until 18:00 and remained relatively stable until the next day at 08:00.Notably, the soil exhibited CH 4 emissions during the daytime (0.0007 to 0.07 mg m -2 h -1 ), while showing fluctuations between emission and absorption during the nighttime (-0.02 to 0.02 mg m -2 h -1 ).During the non-growing season, the soil absorbed CH 4 throughout the day (-0.04 to -0.0007 mg m -2 h -1 ), with fluctuations trend, reaching a peak absorption in the morning before sunrise.

Seasonal variations of soil CH 4 flux in two poplar forests with different soil textures
From April 2019 to December 2020, we conducted 31 CH 4 sampling events at the Sihong poplar plantation, with a daily average soil CH 4 flux of 1.56 ± 0.50 mg m -2 d -1 (Fig. 3).In the Dongtai poplar plantation, we conducted 15 sampling events, resulting in a daily average soil CH 4 flux of 0.70 ± 0.39 mg m -2 d -1 .Both plantations exhibited CH 4 emissions during the growing season and a weak CH 4 uptake during the non-growing season.From April to December 2019, the soil CH 4 flux in both plantations fluctuated slightly and showed a decreasing trend over time.However, in 2020, the soil CH 4 flux in both plantations displayed a unimodal seasonal variation pattern, with a peak occurring between July and September.
A t-test revealed that the soil CH 4 fluxes in the Sihong poplar plantation significantly exceeded those of the Dongtai poplar plantation from August to October and December 2019, as well as July to August and November 2020 (Fig. 4).Repeated measure analysis indicated that, in the Sihong poplar plantation, the soil CH 4 fluxes in April, May, July, and September 2019 were significantly higher than those in June and August.Furthermore, soil CH 4 fluxes in June and August were significantly elevated compared to October and November, with July and August 2020 also displaying significantly higher CH 4 fluxes compared to January, May, June, and September to December.In the Dongtai poplar plantation, the soil CH 4 fluxes from May to July 2019 were significantly higher than those from August to December.Moreover, soil CH 4 fluxes between June and September 2020 exhibited a significant increase compared to January and the period from October to December.

Relationships between soil CH 4 fluxes and environmental factors in two poplar forests
To identify the dominant driving factors influencing changes in CH 4 fluxes between the soil and the atmosphere, we used a GLMM to test the effects of environmental factors on CH 4 fluxes.The result indicated that soil CH 4 fluxes significantly increased with rising Ts (P = 0.010) and SWC (P < 0.001) (Table 1; Fig. 5).However, precipitation did not have a significant impact on soil CH 4 fluxes (P = 0.634).

Discussion
In this investigation, diurnal patterns displayed a single emission peak during the growing season, specifically in its early stages (March and May), occurring between 12:30 and 14:30.During the non-growing season (December), there was a solitary uptake peak detected at 12:30.(Fig. 2).These findings support our first hypothesis regarding diurnal variation trends.However, our results differ from previous observations in upland forests,   which generally indicated an uptake peak around noon during the summer 36,37 (Table 2).Xiao et al. reported a bimodal diurnal CH 4 uptake pattern in a deciduous forest that persisted across all seasons, featuring peaks at 14:00 and 18:00, respectively 38 .Although their diurnal patterns of uptake or emission varied, a commonality among these studies is that diurnal variations are primarily driven by soil temperature.In two other studies, no clear regularity was found during the measurement of diurnal variation in March and May 39,40 .Furthermore, research revealing the diurnal cycles of ecosystems using the EC method also showed various patterns.For example, Querino et al. reported a single emission peak throughout the year in a tropical forest, peaking in the morning after sunrise 41 .In a subtropical forest, Wang et al. observed a singular uptake peak in summer, a lone emission peak in winter, and an absence of a consistent pattern in both spring and fall 42 .Overall, the diurnal cycles exhibited diversity in different ecosystems, driven mainly by temperature.Despite the soil serving as a CH 4 sink during some non-growing season periods, the net emissions during periods of high temperature and humidity in the growing season were substantial enough to compensate for these limited CH 4 sinks.This contrasts with some previous studies that have reported forest soils acting as a CH 4 sink.In some temperate forests, previous studies have revealed that the soil functions as a CH 4 sink, especially showing higher uptake in summer and autumn 36,38,39 .In three subtropical forest ecosystems in southern China, the soil was identified as a CH 4 sink, demonstrating an annual average CH 4 uptake of 3.4 ± 0.9 kg CH 4 -C ha -1 yr -129 .In a recent global synthesis, Feng et al. found that nearly 117 M ha of forest soils emit CH 4 gas with an average magnitude of 0.84 ± 0.83 kg CH 4 -C ha -1 yr -13 .Many previous studies found that higher fluxes generally occurred in summer and autumn 36,38,39 , suggesting temperature plays a critical role in regulating soil CH 4 dynamics.The     1).The soil CH 4 flux significantly increased with rising soil temperature (Fig. 5a).
The soil CH 4 fluxes from the Sihong and Dongtai poplar forests in 2019 did not exhibit a specific seasonal pattern, aligning with earlier research results 29,40 .However, in 2020, the soil CH 4 flux within these poplar plantations displayed a clear unimodal seasonal pattern, with peak fluxes occurring in July-August (Sihong) and July-September (Dongtai), markedly surpassing those observed in other seasons.The seasonal pattern of a single peak observed in our findings is consistent with other studies 36,38,43 .The observed variation in seasonal fluxes between years could potentially be attributed to interannual fluctuations in the summer hydrologic cycle, including factors such as rainfall and soil water content (Fig. 1).The GLMM showed that the soil CH 4 flux significantly increased with rising soil water content (Fig. 5b).The net CH 4 fluxes could be frequently influenced by precipitation and soil moisture conditions 28 .Particularly under the conditions of the Asian monsoon climate, CH 4 emissions could increase with the frequency and amount of summer rainfall 44 .
According to previous studies, soil texture has been considered a strong predictor of CH 4 fluxes 2,45 .In this study, we found that CH 4 emissions from the Sihong poplar plantation, which had a clay loam soil texture, were significantly greater during the periods of August to December 2019 and July to August and November 2020 when compared to the emissions from the Dongtai poplar plantation with a sandy loam soil texture during the same time intervals (Fig. 4).Prior research has highlighted the importance of soil texture in governing greenhouse gas emissions through its influence on the quantity of micro-and macropores within the soil, which is essential for aeration and subsequently affects gas diffusion 28 .Increased clay content could hinder the diffusion of atmospheric CH 4 into the soil, restricting aerobic CH 4 oxidation and enhancing anaerobic microsite CH 4 production in clayey soils, particularly during the rainy season 46,47 .Additionally, Martins et al. highlighted that soil moisture and sand content exerted the most pronounced direct influence on CH 4 fluxes, while rainfall indirectly affected fluxes by directly impacting soil moisture and gas diffusivity 48 .Therefore, in our study, soil texture could be the main factor directly influencing soil CH 4 fluxes, considering the climate conditions at the two sites have no significant difference.
Considering the significant impact of soil texture on CH 4 flux, altering the soil composition within poplar plantations presents a viable strategy for emission reduction.For example, introducing organic matter or adjusting soil composition to enhance drainage and aeration can effectively reduce the creation of anaerobic conditions that foster methanogenesis.Fine-textured soils, such as clay loam, may be amended with coarser materials, such as harvesting residuals, to improve gas diffusion and promote aerobic conditions.Furthermore, modifying soil properties through microbial manipulation has the potential to influence CH 4 flux.Incorporating organic matter, like compost or biochar, can augment microbial activity and foster aerobic conditions.An emerging area of study involves utilizing specific methane-oxidizing microbes to enhance CH 4 uptake.Although this study has shed light on CH 4 spatiotemporal dynamics within poplar plantations, it has certain limitations, such as the focus on only two sites and the use of spark measurements with the chamber-GC approach.Addressing these limitations through extended, long-term monitoring, broader geographical coverage, and more comprehensive factor analysis is necessary to gain a deeper understanding of soil CH 4 flux in plain forests.

Conclusion
Our study contributes to the understanding of spatiotemporal dynamics of soil CH 4 flux in two poplar plantations on plains.At both plantation sites, the soil exhibited net CH 4 emissions throughout the growing season while demonstrating weak CH 4 uptake during the non-growing season.The high CH 4 emissions during periods of high temperature and humidity offset the slight CH 4 sinks.Sandy loam soil displays higher CH 4 absorption rates in winter, whereas clay loam soil exhibits greater CH 4 emissions.On a daily scale, there are significant absorption peaks in winter and significant emission peaks in other seasons in both forests, primarily driven by temperature.On a seasonal scale, the variation of soil CH 4 flux primarily depended on differences in temperature and soil water content.Overall, our study provides valuable insights into multi-time-scale CH 4 measurements on the soil surface and highlights the importance of considering temporal dynamics to reduce the deviations in the CH 4 budget.These findings contribute to improving the accuracy of ecosystem CH 4 budget assessments, aiding in the management and mitigation of greenhouse gas emissions in plain forests.Future studies can build upon these findings to develop more accurate models and strategies for mitigating CH 4 emissions in such forests.

Site description
The study was conducted at two distinct sites: the Sihong poplar plantation (33°19′20′′ N, 118°18′30′′ E) located on the western shore of Hongze Lake, covering an area of 800 ha in the Huang-Huai Plain, and the Dongtai poplar plantation, covering an area of 3,000 ha (32°51′26′′ N, 120°51′01′′ E), a significant coastal protection forest in the middle and lower reaches of the Yangtze River Plain, China 49 .The four seasons of spring, summer, autumn, and winter in these two research areas refer to March to May, June to August, September to November, and December to February of the following year, respectively.The Sihong poplar plantation is bordered by water on three sides and falls within the bounds of a characteristic subtropical monsoon climate.The area has a mean annual sunlight duration of 2,327 hours, a mean annual temperature (MAT) of 14.1 ℃, and a mean annual precipitation (MAP) of 897 mm.The soil is classified as Hongze Lake silted soil, identified as Gleysols following the IUSS World Soil Classification 50  The Dongtai poplar plantation is situated along the coastline of the Yellow Sea and falls within a marine monsoon climate zone.It has an average annual sunshine duration of 2,209 hours, a MAT of 14.6 ℃, and a MAP of 1,050 mm.The soil in this region is classified as desalinated meadow soil, categorized as Fluvisols according to the IUSS World Soil Classification 50 , and is characterized by a sandy loam texture.The soil consists of 160, 720, and 120 g kg -1 of silt, sand, and clay, respectively.The Dongtai poplar plantation, initiated in 2006, consists of a single-cultivar plantation utilizing P. canadensis cv.'I-72/58' .The understory vegetation consists of a few species of shrubs dominated by Morus alba and Broussonetia papyrifera, as well as herbs including Solidago canadensis, Cayratia japonica, Microstegium vimineum, Carpesium abrotanoides, Achyranthes bidentata, and Rostellularia procumbens.We did not intervene or manage litter in the two forests during the entire experimental period.
Three sample plots were established as replicates at both sites, respectively, with each plot measuring 60 m × 120 m in Sihong and 60 m × 60 m in Dongtai (Fig. 6a,b).In March 2019, before gas sampling, five points were chosen per block using an S-type pattern to collect soil samples at a 0-20 cm depth for measuring soil physical and chemical characteristics.Soil samples from the five points within each block were uniformly blended to create an independent sample.The fresh soil samples were brought to the lab in sterile fresh-keeping bags and reserved at 4 ℃.After removing plant roots and small stones, some samples were left to air-dry naturally and subsequently sifted using a 2 mm mesh for further detection.Soil bulk density (BD) was assessed using the core method, while soil water content (SWC) was measured through the oven drying method (24-hour drying process at 105 ℃).Soil pH was determined by a pH meter (AB15 + Basic; Accumet, San Diego, CA, USA) with a 1:2.5 soil-to-water ratio, while soil organic carbon (SOC) was measured by potassium dichromate oxidation-ferrous sulfate titration.Total nitrogen content (TN) was measured using an automatic continuous flow analyzer (AA3; Bran Luebbe, Norderstedt, Germany) after digesting 1 g of dry soil with a catalyst (2 mL) and H 2 SO 4 (5 mL).Ammonium nitrogen (NH 4 + -N) and nitrate nitrogen (NO 3 − -N) were determined through ultraviolet-visible (UV-vis) spectrophotometry (UV-2550; Shimadzu, Tokyo, Japan) following the extraction of 5 g of soil in 50 mL of 2 M KCl.Fifteen trees in each block were selected to measure tree height and diameter at breast height (DBH).Tree height was measured using vertex laser rangefinders (Haglöf, Langsele, Sweden), while DBH was measured by a caliper.The stand characteristics and basic soil physicochemical properties of the Sihong and Dongtai poplar plantations are detailed in Table 3. Table 3. Stand characteristics and soil properties (0-20 cm) in the two investigated poplar plantations (mean ± SD, n = 3).DBH, Diameter at breast height; BD, Bulk density; SWC, Soil water content; SOC, Soil organic carbon; TN, Total nitrogen; NO In March 2019, three random sampling points were selected within each block to collect soil CH 4 gas samples in both plantations, respectively.Three cylindrical static chambers constructed from polyvinyl chloride (PVC) were randomly installed in each block in the Sihong and Dongtai poplar plantations, resulting in a total of 9 static chambers installed in each plantation (Fig. 6a,b).The static chamber system includes a base (30 cm inner diameter, 35 cm outer diameter, and 15 cm height) and a chamber (30 cm in both diameter and height).The base was inserted vertically into the ground to a depth of 10 cm, leaving 5 cm above ground, thereby creating a closed ring with distinct inner and outer diameters (Fig. 6c).During CH 4 gas sampling, water was added to the rings to establish an effective seal between the base and the chamber (Fig. 6d).A stand (15 cm height) equipped with a temperature recorder (DS1923; Wdsen Electronic Technology Co. Ltd, Shanghai, China) was installed in the center of the base during the gas sampling period for calculating CH 4 fluxes.Gas sampling was carried out at the center of the chamber top through a rubber stopper with a hole, using a syringe (18-gauge needle, 15 cm long) connected to a T-joint.Specifically, from April 2019 to December 2020, gas sampling was conducted respectively for 9 chambers at the Sihong and Dongtai poplar plantations between 8:00 and 12:00 am on days without rain or snow.During each sampling day, we gathered 30 mL gas samples at intervals of 0, 20, 40, and 60 minutes and subsequently transferred them into pre-evacuated gas bottles for storage.At the Sihong poplar plantation, gas sampling frequency ranged from 1 to 4 times per month, resulting in a total of 31 on-site sampling activities and the collection of 1,116 soil gas samples.Due to traffic and accessibility constraints at the Dongtai poplar plantation, gas sampling was conducted once per month, resulting in 15 on-site sampling activities and the collection of 540 soil gas samples.Data collection was suspended from February to April 2020 owing to the impact of the COVID-19 pandemic.
Gas samples were transported to the lab and subsequently examined for CH 4 concentration using a gas chromatograph (GC; 7890B, Agilent Technologies, Inc., Palo Alto, CA, USA).The GC used in the study was equipped with a pair of Porapak Q columns (each measuring 1.83 meters in length, with a 2 mm inner diameter and 80/100 mesh) alongside a flame ionization detector (FID).The column oven and detector were operated at temperatures of 60 ℃ and 250 ℃, respectively.We employed ultra-high purity nitrogen (N 2 , 99.999%) as the carrier gas, flowing at a rate of 30 mL min -1 .Moreover, hydrogen (H 2 ) and high-purity air (99.999%) served as the fuel and auxiliary fuel gases for the FID, with flow rates set at 40-and 400-mL min -1 , respectively.The instrument was calibrated with a standard gas (10.2 ppm) both before and after each measurement.The formula for calculating soil CH 4 flux 51 (F, mg m -2 d -1 ) is as follows: Here, ρ represents the CH 4 density under standard conditions (g L -1 ); V represents the volume of the static chamber (cm 3 ); A denotes the surface area encompassed by the static chamber (cm 2 ); and T stands for the temperature inside the static chamber during sampling moment (℃).Additionally, ∆C/∆t represents the linear slope of CH 4 concentration change within the chamber over time (ppm min -1 ), with an R 2 ≥ 0.9 considered valid for further analysis.The limit of detection (LOD) of CH 4 flux for the GC system was 0.49 ppm, calculated using methodologies provided by Minamikawa et al. 52 .
Moreover, air temperature (Ta) was measured using a platinum resistance thermometer (PRT) installed on a flux (CO 2 and H 2 O) tower.Precipitation was recorded with a tipping bucket rain gauge installed at the base of the flux tower.For missing meteorological data, we gap-filled using data from the National Meteorology Information Center (NMIC) of China.Soil temperature (Ts) and SWC were measured using thermocouples and time domain reflectometry (TDR, Trime-EZ, IMKO) probes, respectively, installed at a depth of 10 cm.

Measurement of diurnal variation of soil CH 4
In 2019, we employed a swiftly deployable chamber system utilizing laser technology to measure diurnal variations in soil CH 4 flux, using the Los Gatos Research (LGR; ABB, Canada) instrument.The recording frequency was set to 2 Hz (Fig. 6e).Monitoring took place during different growth stages of the poplar, including the early growing season (March-April), the rapid growing season (May-June), the late peak growing season (August-September), and the non-growing season (November-December).The measurement events were conducted on days without rain or snow in different seasons.Considering the complexities of field sampling, including the daytime heat, nighttime cold, and threats from certain animals (e.g., snakes), we selected a static chamber at each site to measure the diurnal variation of soil CH 4 flux.The sampling started at around 08:00 am and ended the following day at around 08:00 am, with the static chamber sampled for 15 min each time.The CH 4 concentration change rate per unit of time was calculated using the concentration values during the stable 10-minute period after excluding the first 3 min and the last 2 min of CH 4 concentration values, with an R 2 ≥ 0.9 considered valid for further calculating fluxes.There should be at least a 15 min interval between each measurement activity to ensure sufficient battery power and prevent the instruments from overheating and shutting down.The minimum detection limit (MDL) of the ultraportable LGR CH 4 analyzer is 0.002 ppm, with an accuracy of less than 1% 53 .

Statistical analysis
A repeated measures analysis of variance was used to examine variations in soil CH 4 flux across different periods.Subsequently, a T-test was performed to evaluate the difference in soil CH 4 flux between the Sihong and Dongtai poplar plantations for the same month.When the number of repetitions differed between the two groups, specific data processing steps were taken to ensure the validity of the comparison.For the Sihong site, where CH 4 flux www.nature.com/scientificreports/measurements were conducted two or four times in a month, the average of these replicates was calculated for each month and considered as a single monthly CH 4 flux value.For the Dongtai site, where CH 4 flux measurements were conducted only once per month, this single measurement served as the monthly CH 4 flux value.A generalized linear mixed model (GLMM) was used to assess the effect of climate and soil factors on soil CH 4 flux, incorporating precipitation, Ts, SWC, and sampling site (SH and DT poplar forests) as fixed effects and sampling points (static chambers) as a random effect.All numerical variables were standardized (subtracting the mean and then dividing by the standard deviation) to enhance the likelihood of model convergence 54 .We used the R package "performance" for model diagnostics to verify the normality of residuals and the multicollinearity of variables 55 .Variables displaying correlation coefficients exceeding 0.7 were eliminated from the analysis 56 .Unless otherwise stated, the statistical analysis was conducted at a significance level of 0.05.All statistical analyses were conducted using R software version 4.3.0 57. Vol

Figure 1 .
Figure 1.Monthly averages of air temperature, cumulative precipitation, soil temperature, and soil water content in Sihong (a) and Dongtai (b) poplar plantations.SH, Sihong; DT, Dongtai.Ta, Ts, and SWC represent air and soil temperature and soil water content.

Figure 2 .
Figure 2. Diurnal variation of soil CH 4 fluxes in the Sihong (a-d) and Dongtai (e-f) poplar plantations at different growing periods in 2019.Positive and negative values of CH 4 flux represent the CH 4 emission and uptake, respectively.The triangles represent the flux values marked at 2-hour intervals.SH, Sihong; DT, Dongtai.

Figure 3 .
Figure 3. Daily average soil CH 4 flux at different sampling days during the study period April 2019-December 2020 in the Sihong and Dongtai poplar plantations.Positive and negative data of CH 4 flux stand for the CH 4 emission and uptake, respectively.Error bars represent the standard error.Data from February 1 to April 30, 2020, was not gathered due to the COVID-19 pandemic.SH, Sihong; DT, Dongtai.

Figure 4 .
Figure 4. Spatiotemporal differences of monthly average soil CH 4 flux in the Sihong and Dongtai poplar plantations.Positive and negative data of CH 4 flux stand for the CH 4 emission and uptake, respectively.The capital and lower-case letters on the error bars represent respectively the significant difference (at P ≤ 0.05) of soil CH 4 fluxes in the Sihong and Dongtai poplar plantations at different months.Asterisks indicate the significant difference in soil CH 4 fluxes between the Sihong and Dongtai poplar plantations in the same month (* 0.01 < P ≤ 0.05, ** 0.001 < P ≤ 0.01, * P ≤ 0.001).Error bars represent the standard error.Data from February 1 to April 30, 2020, was not gathered due to the COVID-19 pandemic.SH, Sihong; DT, Dongtai.

Table 1 .
Statistical summary of generalized linear mixed model analysis of the effect of climatic and soil factors as fixed effects and soil static chambers as random effect on soil CH 4 fluxes.Numeric variables such as daily average of Ts, SWC, and precipitation were standardized, that is, subtract the mean and then divide by the standard deviation.The bold P-values indicate the significant impacts of fixed factors on soil CH 4 fluxes (P < 0.05).Ts, soil temperature; SWC, soil water content.SH, Sihong; DT, Dongtai.

Figure 5 .
Figure 5.The effect of soil temperature (a) and soil water content (b) on the soil CH 4 flux.The light gray shading indicates the 95% confidence interval.Ts, soil temperature; SWC, soil water content.

Table 2 .
Comparison of the diurnal and seasonal patterns of methane flux for various different forest types.